In the drafting of line drawings such as employed for parts layout, architectural renderings and the like, a variety of specialized tools commonly are employed. First of all, a drafting board or table with a straight edge or equivalent is generally used. Then, an assortment of drafting tools such as T-squares, rules, compasses, protractors, and various triangles of different angles are employed. When a draftsman uses such tools in laying out a drawing, much time is spent in picking up and setting down the various tools used at various times to draw lines at different angles, to draw circles, and the like. In addition, since these various drafting tools, particularly as used by professional draftsmen and architects, must be accurate, they are relatively expensive.
Even with the variety of conventional tools which are mentioned above, the ability to quickly and accurately perform many functions required of a draftsman often is not facilitated by the tools themselves, but the draftsman must make various mental calculations. This is particularly true in deciding which of various standard triangles are to be used for laying out angles for a given reference line or the like. For odd angles, a separate protractor usually is employed to establish points which then are connected together using a straight edge to form lines at the desired angles.
Basically, most drafting work for layout drawings comprises straight lines at various angles to one another. Thus, the common approach is to use T-squares (or their equivalents), triangles of various angles, and protractors. To replace all or some of these tools with a single multi-purpose tool has been attempted in the past.
The Elger U.S. Pat. No. 3,289,299 is directed to a protractor having a configuration in which the protractor itself is printed or etched onto the face of a trapezoidal-shaped base member. The top of the trapezoid is parallel to the bottom of the trapezoid which, in turn, serves as the base line for the protractor. The top and bottom are interconnected by converging oblique sides which make an angle of 60.degree. with respect to the base line of the protractor. This is an angle which is commonly encountered in drafting so that the outside edges of the tool may be used directly to lay out lines at this angle. Also scribed on the face of the protractor are a large number of concentric circles each having a small hole on it to permit use of the tool to draw arcs or circles having the desired radii marked on the tool adjacent each of the concentric lines. The tool of this patent is capable of somewhat specialized use as a protractor but still requires conventional picking up and setting down of the tool to draw lines at various angles to one another from the point of origin of the protractor. The drafting tool of Elger further has a mounting block mechanism attached to it at the origin of the protractor to permit specialized use of the tool in drawing circles of various diameters. As a result, however, the origin point of the protractor is encircled with this additional apparatus so that it is not possible to directly strike a line from the origin point to some other point while this apparatus is in place.
Protractor tools which also may be used as straight edge rulers are disclosed in the patents to Bryson, U.S. Pat. No. 846,006, issued Mar. 5, 1907; and Owens, Jr., U.S. Pat. No. 1,808,705, issed June 2, 1931. The patent to Bryson discloses an elongated ruler having both longitudinal and perpendicular transverse spaced lines on it. In addition, a protractor with its origin near the center of one of sthe edges of the rule is provided and lines at various angles also are placed on the face of the ruler to facilitate its use as a layout drafting tool. The various lines on this tool permit it to be used to draw a variety of straight lines at various angles with respect to one another so that it greatly facilitates many layout operations. The tool, however, requires accurate placement or alignment of the origin of the protractor with a given point in order to strike lines at different angles from that point.
The Owen, Jr. patent is similar to the ruler of Bryson except that it has a rotatable eyelet mounted in it at the center point of circles which may be drawn by placing a pencil through a hole located at a measured distance from the eyelet. The tool itself is rectangular and has a protractor scribed on the face of the rule. The origin of the protractor is at the center point of one of the edges of the tool. Once again, however, accurate alignment of the origin of the protractor with a point on a line is required by a draftsman using the tool in order to strike a line from that point at some other angle.
It is desirable to provide a drafting tool for use in making a variety of layout drawings which overcomes the disadvantages of the various tools of the prior art. In addition, it is desirable to provide a variable angle straight edge which quickly and accurately coordinates the work of the draftsman in locating the origin of a protractor at a point on a line without the minute hand adjustments which normally are necessary in the use of conventional protractors and/or straight edges.